1. Field of the Invention
The present invention relates to a method of and an apparatus for converting the gradient of image data to produce output image data from input image data, in which visibility of banding is improved.
2. Description of the Related Art
Image data that are produced by an image input device such as an image scanner or the like are finally outputted as a visible image from an image output device. For outputting such image data as a visible image, it is necessary to calibrate the image data because the image output characteristics of the image output device generally vary from device to device, or depending on the type of the image output device.
FIG. 10 of the accompanying drawings shows a gradient conversion table for converting input image data to output image data. The converted, i.e., calibrated, output image data in a region A are shown in FIG. 11A of the accompanying drawings, whereas the converted or calibrated output image data in a region B are shown in FIG. 11B of the accompanying drawings. The calibrated output image data in these regions A, B are plotted respectively in FIGS. 12A and 12B of the accompanying drawings.
When output image data are thus calibrated, no smooth conversion may be effected between the input and output image data, resulting in so-called banding or a density jump. Stated otherwise, when 8-bit input image data having 256 density steps are converted into 8-bit output image data, for example, the 256 density steps may not virtually be obtained.
According to one known solution disclosed in Japanese laid-open patent publication No. 1-123373, N-bit image data are gradient-converted to M-bit image data (N&lt;M), and the image data of the decimal part (low-order (M-N) bits) of the M-bit image data are compared with (M-N)-bit random-number data. If the image data are larger than the random-number data, then "1" is added to the least significant bit of the integral part of the M-bit image data, and the resultant image data are outputted to produce an image with a banding of reduced visibility.
However, since only the least significant bit of the M-bit image data is controlled, any banding is not sufficiently improved. If the banding is large, the image data may not fully been calibrated.
Where a halftone-dot image is to be produced from 8-bit input image data having 256 density steps by an image output device, the number of density steps of the halftone-dot image depends on the resolution and screen ruling of the image output device. For example, if the resolution is 1,200 dots/inch (dpi) and the screen ruling is 100 lines/inch, then since one halftone dot is composed of 144 (12.times.12) dots, only 144 density steps can be produced even if the output image data are of 8 bits. In this case, the calibration of image data according to the above conventional process fails to reduce the visibility of the banding sufficiently since the scattering effect produced by the random number is poor, even though it is possible to obtain 256 density steps in the output image data.
Further, the aforementioned conventional art does not teach that when N-bit input image data having a number of density step "a" is gradient converted to M-bit image data (N&lt;M), the number of density steps is converted to "b" which is different from the number of density step "a" of the input image data (a.noteq.b) depending on a resolution and a screen ruling of an image output device, and at the same time, the number of density step "a" of the input image data is substantially secured in the output image data.